A two-component developer used in electrophotography is generally a mixture of toners and carriers. Carriers are used to give toners an appropriate amount of electrostatic charge of suitable polarity.
Resin-coated carriers that are prepared by coating the surfaces of carrier core particles with a resin is advantageously employed due to its improved durability and frictional chargeability.
The spray-coating method, an example of a wet method, has been widely employed to provide a resin coating layer oh the surface of a core particle. However, by this method, resin particles are likely to agglomerate, resulting in difficulty in obtaining carriers with a prescribed size distribution in a high yield. This method also has such a defect as a prolonged production time.
The following are the methods other than the spray coating that have been proposed to solve the above problems:
1. Coating by the dry method the surface of a core particle with resin particles of which the particle sizes are not more than 1/10 of the core particle (disclosed in Japanese Patent Application Open to Public Inspection No. 235959/1988). PA1 2. Coating by the dry method the surface of a core particle with resin particles at a temperature higher than the melting point of the resin particles (disclosed in Japanese Patent Application Open to Public Inspection No. 35735/1979). PA1 3. Heating metal core particles with specific surface areas of 200 to 1300 cm.sup.2 /g at 160.degree. to 343.3.degree. C. for 20 to 120 minutes using 0.05 to 3.0% by weight of elementary particles with particle sizes of 0.1 to 30 .mu.m (disclosed in Japanese Patent Application Open to Public Inspection No. 118047/1980). PA1 4. Coating by the dry method the surface of a core particle with resin particles with an average particle size of not more than 1 .mu.m (disclosed in Japanese Patent Application Open to Public Inspection No. 27858/1988). PA1 5. Forming a layer of polymeric fine particles on the surface of a core particle, and solidifying it (disclosed in Japanese Patent Application Open to Public Inspection No. 37360/1988). PA1 BET specific surface areas of 5 to 150 m.sup.2 /g; and PA1 a volume average particle size of 1.5 to 5.0 .mu.m.
In the preceding methods 2, 3, 4 and 5 where resin particles being in contact with the surface of a core particle are melted forcibly, they are likely not only to stick to each other but also to help core particles stick to each other, thus making it difficult to obtain resin-coated carriers with a prescribed size distribution in a high yield. These methods also have problems that a prolonged cooling time is needed since a resin layer is formed at high temperatures, and that the surface of a resin coating layer becomes uneven, since part of a resin film tends to peel off when agglomerated core particles are crushed to increase the yield. The unevenness of a resin coating layer makes the frictional chargeability of a carrier unstable at high temperature and humidity.
By the method 1, it is hard to obtain a carrier with a uniform resin coating layer, since the spreadability and film-forming property of resin particles are poor due to their large particle sizes.
Another dry method was proposed. In the method a magnetic particle is coated with a resinous substance which comprises adding to magnetic particles with a weight average particle size of 10 to 200 .mu.m resin particles of which the weight average particle size is not more than 1/200 of that of the magnetic particles to form a uniform mixture, and giving impact to this mixture repeatedly in a mixer of which the temperature is set in the range of 50.degree. to 110.degree. C. (Japanese Patent Application Open to Public Inspection No. 87168/1990).
However, this method has been found to have a problem that the handling of resin particles is difficult due to their extremely small sizes. For instance, resin particles are likely to fly during the production process, making sufficient mixing difficult. Further, when coating is performed by the dry method in a mixer having a rotator, where air purge is usually done to protect the sealed portion of a bearing, resin coating efficiency, i.e. the weight ratio of resin particles that are formed into a layer to those as raw material, decreases due to serious fly loss of resin particles.
Due to such low resin coating efficiency, considerable amounts of resin particles or agglomerated resin particles are allowed to remain on the surface of a carrier in a free state without forming a film (these particles and agglomerated particles will often be referred to as "white powder"). Such white powder tends to stick to the surface of a resin-coated carrier electrostatically, and hinder the frictional charging of carriers and toners, making toners charged only weakly. This phenomenon causes fogging at the early stage of forming an image.
When a large amount of white powder is present on the surface of a resin-coated carrier, it tends to transfer to a light-sensitive element selectively at the time of developing, affecting adversely developing and cleaning conditions. That is, since white powder has a charging polarity opposite to that of a toner, it selectively sticks to the non-image-forming portion of a light-sensitive element, and is sent to the cleaning portion without being transferred. This leads to the overloading of the cleaning portion, and then to insufficient cleaning. If cleaning is insufficient, the surface of a light-sensitive element is subjected to filming. As a result of this, the light-sensitivity of a light-sensitive element is lowered, causing an image to be fogged.